1. Field of the Invention
The present invention relates to a diffused light controlling optical sheet suitable for use in controlling a back light in a liquid crystal display, a liquid crystal color television set and the like, and relates to a back light device and a liquid crystal display apparatus which use the same optical sheet.
2. Prior Art
Up to now, there has been a back light device for irradiating light on the back of a liquid crystal display apparatus in order to improve the visibility of a liquid crystal display screen.
In order to improve the illumination efficiency of an illumination light (back light) from such a back light device, it is necessary to improve the directivity of the illumination light (back light) toward a user.
However, in case that the directivity of an illumination light (output light) is too intense, there is a problem that the angle of view becomes very narrow and the brightness of a screen is sharply reduced by just slightly moving user""s point of view.
In order to cope with such a problem as this, as disclosed in a Japanese laid-open publication Tokkaihei No. 6-148408 for example, a light control sheet has been proposed which is provided with both of an optical condensability for giving a sufficient brightness in the direction of illumination and an optical diffusibility for bringing a uniformly bright visibility within a certain range of the angle of view.
This light control sheet has one face which is a structural surface having macroscopically a regularly-wavy corrugated pattern or a corrugated pattern in which a number of ridges each of which has a substantially triangle-shaped cross section are formed in parallel with one another and microscopically a corrugated pattern which a fine-granulation process or a light-diffusion coating process is applied to, and the other face which has an optical plane or a plane to which a fine-granulation process is applied.
A light control sheet disclosed in the above-mentioned Japanese laid-open publication Tokkaihei No. 6-148408 has a problem that its optical diffusion effect given by a surface granulation process or coating process is excessive and the directivity of its output light is deteriorated.
In addition to this, a technique of outputting a light of directivity by applying a refractive effect of a sheet whose cross section is of a triangular-prism shape has been disclosed in a Japanese laid-open publication Tokkaisho No. 63-318008 and similar technical ideas have been disclosed in Japanese laid-open publications Tokkaihei No. 2-257188 and Jikkaihei No. 3-69184 and moreover as a method for further enhancing the directivity of an output light a method of using two corrugated sheets has been disclosed in a Japanese patent publication Tokkouhei No. 1-37801.
However, any of techniques disclosed in the above-mentioned Japanese laid-open publications Tokkaisho No. 63-318008, Tokkaihei No. 2-257188 and Jikkaihei No. 3-69184, and Japanese patent publication Tokkouhei No. 1-37801 has a problem that it cannot sufficiently enhance the directivity of an output light and cannot control a direction showing the peak in directivity.
As disclosed in a Japanese laid-open publication Tokkaihei No. 2-84618, there is a method of applying the total reflection of a film having triangular prisms in cross section as a method for controlling a direction showing the peak in directivity of an output light.
However, this method is a method of outputting at a specific angle only a light ray inputted at a specific angle, and has a problem that the incident angle of a light ray is limited and it does not control the directivity itself of a diffused light and therefore cannot control at the same time a direction showing the peak in directivity of a diffused light and the directivity itself of the diffused light.
The present invention has been developed in consideration of the above-mentioned problems, and an object of the present invention is to provide a diffused light controlling optical sheet, a back light device and a liquid crystal display apparatus which make it possible to further enhance the directivity of a diffused light generated by diffusing an output light and to control a direction showing the peak in directivity of the diffused light.
The present invention achieves the above-mentioned object by using a diffused light controlling optical sheet which is made of a light-transmissive material and has at one side a light receiving surface of a plane shape and at the other side a serrulated surface which has a plurality of convex parts for converting a light inputted through the said light receiving surface to a diffused light of intense directivity and in which each of the said convex parts is provided with side faces extending from its top to its bottom and each of said side faces of said convex part is formed into a plane making an angle of 60 to 90 degrees with the said light receiving surface.
Said diffused light controlling optical sheet may have a fine material, which is slightly different in refractive index from said light-transmissive material, disposed in said convex parts and may have a light diffusion effect by means of a refractivity difference between said fine material and said light-transmissive material.
Said diffused light controlling optical sheet may have said fine material which is in particle diameter more than 0.5 xcexcm and less than 10 xcexcm, and preferably 1 xcexcm or more and 5 xcexcm or less.
Said diffused light controlling optical sheet may have said convex part provided with said side faces of fine ruggedness.
Said diffused light controlling optical sheet may have said convex part, wherein the centerline average roughness of said fine ruggedness is 0.5 xcexcm or more and less than 10 xcexcm, preferably 1 to 5 xcexcm, and more preferably 2 to 3 xcexcm.
Said diffused light controlling optical sheet may have said convex part provided with said side faces coated with a light diffusing material.
Said diffused light controlling optical sheet may have said light diffusing material comprises in a base a fine material which is different in refractive index from this base and is in particle diameter 0.5 xcexcm or more and 10 xcexcm or less, and preferably 1 to 5 xcexcm.
Said diffused light controlling optical sheet may have said convex parts disposed at intervals of 1 to 500 xcexcm.
Said diffused light controlling optical sheet may have said convex parts each top of which is in the shape of a plane parallel with said light receiving surface.
Said diffused light controlling optical sheet may have the top of said convex part made into a convex lens part.
Said diffused light controlling optical sheet may have said convex parts in which the half width of intensity of its output light is within an angle range of 24 degrees or less.
Said diffused light controlling optical sheet may have a fine material slightly different in refractive index from said light-transmissive material distributed inside said convex parts and a light diffusion effect is obtained by a refractivity difference between said fine material and said light transmissive material, and said side face of said convex part is provided with fine ruggedness.
Said diffused light controlling optical sheet may have said convex parts disposed at intervals of 1 to 500 xcexcm.
Said diffused light controlling optical sheet may have a fine material slightly different in refractive index from said light-transmissive material distributed inside said convex parts and a light diffusion effect is obtained by said refractivity difference between said fine material and said light-transmissive material, and a light diffusing material is coated on said side faces of said convex parts.
And the present invention achieves the above-mentioned objects by means of a back light device which is provided with a plane-shaped light source and has said diffused light controlling optical sheet arranged so that its light receiving surface faces the light outputting surface of said plane-shaped light source with an air layer between them.
Furthermore, the present invention achieves the above-mentioned objects by means of a liquid crystal display apparatus which is provided with such a back light device as described above and uses a light outputted from said back light device as a back light.
In the present invention, since each side face of the convex parts at the light outputting side of the diffused light controlling optical sheet is made into a plane making an angle of 60 to 90 degrees with said light receiving surface, a light inputted through the light receiving surface is intensely refracted in the front direction and thereby the directivity of an output light can be enhanced.